Electropolishing composition and process



l atnted Feb. 20 1951 ELECTROPOLISHING COMPOSITION AND PROCESS William J. Neill, Columbus, Ohio, assignor to Columbus Metal Products Inc., Columbus, Ohio,

a corporation of Ohio No Drawing. Application January 7, 1948, Serial No. 1,075

4 Claims. (Cl. 204-1405) The inventions disclosed in this application relate to compositions of matter and to processes of polishing of metals. The process inventions are especially applicable to and are illustrated by processes useful in the polishing of stainless steels. The compositions disclosed are especially useful in such polishing operations.

Anodic methods of electrolytically cleaning metals have been known heretofore. While obtaining a lustrous finish on stainless steel by mechanical methods requires difficult and costly grinding and buffing operations, a mirror like finish may be obtained by electrolytic polishing, without the use of any mechanical methods and at a lower cost. Mechanical polishing of stainless steel, moreover, sometimes causes a stressed condition in which the metal is formed into scratched troughs and ridges. Essentially, the electrolytic process consists of making the article to be polished, the anode, in an acid electrolytic bath capable of forming a soluble salt with the constituent metal. The cathode can be of any suitable conductive inactive material. The current density should be sumcient to form and maintain a substantially or partially solid oxidation product on the anode (the article being polished) and permit such oxidation product to have viscous flow from the surface of the article being polished. That is to say, the oxidation product forms a film on the surface of the material being polished. This film increases the resistance to the flow of the current, the film being thicker in the low points of the material being polished than at the high points. The resistance to the flow of the current is greater at such low points and the electrolytic action is thus greater at the high points. On the one hand, if no film at all is formed, the electroytic attack on the material is rapid and not preferential and forms an undesired etched surface. On the other hand, if a film is formed, the film usually builds up to an undesirable thickness over all parts of the material being polished with a resultant loss in efliciency of the polishing process.

I have discovered that with certain types of polishing compositions, it is possible to obtain a more eflicient polishing effect. With the use of my preferred compositions there is cyclic building up and lowering of the film so that there is a cycle of high and low current flow. While it is necessary that at least a thin film be provided in order to prevent too rapid attack on the metal and such non-preferential attack that the result is an etched surface, yet it is also of great advantage to be able to reduce periodically the film so as to be able to obtain greater efficiency in the anodic polishing. With my solutions and process in which the film periodically thins out and is again built up, smooth and bright surfaces can be obtained with less current consumption and time.

One of the objects of my invention, therefore, is the provision of a process for the anodic p01- ishing of metal by which smooth and bright surfaces can be obtained with less current consumption and in a shorter time.

A further object of my invention is the provision of more eflicient compositions for the anodic polishing of metals.

Further objects and advantages of my invention will be apparent from a reading of the subjoined specification and claims.

In general the preferred process described herein in illustration of my invention comprises immersion and preferably substantia ly continuous agitation of the material to be cleaned in an electrolytic solution consisting of one of my compositions: it comprises also, passing an electrical current from the material to be cleaned (as an anode) through the solution and to a suitable cathode which is also immersed in the solution to form a film on the work to be cleaned, the film being effective to alternately decrease the current flow so as to cause an alternate thinning out of the film and to rebuilding thereof which efficiently removes the high points from the material being cleaned and polishes such material with a minimum consumption of electricity and in a minimum time.

In operation, the material to be polished is suitably cleaned of grease, oil, etc. by immersion or washing in alkaline solutions or vapor degreasers of the trichlorethylene type. Actually, cleaning need not be as thorough as for electroplating. Scale such as welding scale, bloom, etc., need not be removed as the electropolishing takes care of them. The racks upon which the parts are placed are insulated all over to prevent dissolution of the rack. Contact areas are left bare of course. Contacts should be firm and of suitable area to carry the current necessary for polishing. Poor or loose contacts result in burned areas and irregular finish on the parts being polished.

The racks with the parts to be polished are then immersed in the electrolytic solution and firm contact made to the anode bar which is the positive side of the circuit. Spring clamping, bolting or some other method of making firm contact is desirable at this point as agitation of MATERIALS TO BE POLISHED Various kinds of metals including various steels. nickel, and nickel electroplates can be polished by my processes. I have found that the processes are especially suitable for the polishing of stainless steels of the nickel grades such as 18-8, 302, 7304-321 and that the processes work exceptionally -=.well for the polishing of titanium bearing grades such-as 321.

COMPOSITION OF ELECTROLYTIC SOLUTION For reasons which I will explain subsequently,

.Irprefer to use electrolytic solutions having desirable current varyingcharacteristics when used *withthe particular material being cleaned. For polishing stainless steels, I prefer to use a composition which includessulphuric acidyfiuoboric :acid (orsources thereof) andwater. Oxalic acid maybe included optionally. Preferably compositions consist of 40-65% sulphuric acid; 5?20% rfluoboric acid; 25-55%water.

.Example I 45%Sulphuric acid 66 degree'Baum (an aqueous solution of about 93% acid and 7% water) 13% Fluoboric acid commercial (an aqueous so- :lution of about 26% acid) 42% Water (The Baum of this-solution is -about 42) Example II 2.5 gal. Sulphuric acid (as above) '2 gal. Water 1000 cc. Zinc fluoborate 1800 cc. Fluoboric acid (as above) 40 grams Oxalio acid (crystals) "ExampZe'III 48% Sulphuric acid (as above) 14% Fluoboric acid (as above) 37% Water 1% Oxalic acid (crystals) .Solutions containing as much as 65% sulphuric acid with corresponding decrease in water percent .are practical. Fluoboric acid can be varied from 5% to.20% with corresponding decrease in percent of water.

-Increases in sulphuric-acid, tend to lower inter- -..nal resistance ofsolution. ,Increasesin percent of water tend to increase resistance.

There .is: comparatively nochange in resistance caused by fluoboricv acid variations within the limits men-- tioned.

gi ig and nickeifiuoborates can be used instead of fluoboric 'acid iif'percents giving the same amounts of fluoboric acid, and zinc fiuoborate seems to have some benefit in causing the oathodes to be coated with a film that allows sludge formation to fall off more easily.

Oxalic acid crystals can be added to the amount up to 130 grams per gallon.

The addition of oxalic acid seems to smooth the finish to some extent and has a stabilizing influence on the bath. The fluoboric acid is very desirable as an agent which provides for the necessary formation of the varying film against the anode which, through its resistance, permits preferential attack on the high points of the material being polished.

VARIATION OF CURRENT I prefer to use electrolytic compositions coming within the limits set out above. With the use 15 of such compositions, it appears that as the cur- .rent builds up, the ionization action rapidly attacks the metal by causing oxidation of the metal and the formation of such oxides and salts as iron oxides and sulphates, chromium oxides and sulphates and probably some other salts such as nickel sulphates and iron borates, etc. These oxides and salts going into solution form -a'fairly thick film around the work piece which insulates the material being polished to a certain extent and prevents or minimizes further rapid attack thereon at the time. It appears that because of the resistance the current is then reduced so that the film either thins out or drops away from the material being polished. The ourrent is then automatically increased and again rapidly attacks the material. This process is continuously repeated until the material is'adequately polished.

I have thus found that with the use of the .compositions described above, I can automatically decrease and increase the current being used. 'It is a peculiarity of certain solutions such as, for example, a solution of sulphuric acid, fluoboric acid and water within the percentages specified that the current flow is automatically increased and decreased. Thus, a peculiarity of all the foregoing solutions is that there is a definite ..cycle of film formation and dissolution. This cycle is indicated by an increase in the resistance of the circuit. There is a peak and then ..a drop in resistance, then the cycle repeats. To equotefrom my notes on this observation:

f65. degrees C., 180 amps sq..ft. 3.75 volts. -The amperage cycle at this setting is about .5 seconds with a 15 amp variation between high and low.

To summarized, the usual theory of anodic cleaning of stainless steel is as follows: When the stainless steel is made the anode, in the polishing solution, and'the contact made, a'film :forms at the interface of the solution and the material being polished. The anode being agitated mechanically, the wiping action of the solution across the face of the material keeps the film thinner on the high points and allows it.to

build up in the microscopic recesses. As the re- ..sistance of this film is in direct ratio to its thickness, the action allowsmore rapid attack of the ..base-smetal .on the .high pointswhere thefilm isthinnest and so wears them off, a sort of level- ,-,iI 1g process.

1011 the other hand, in our processes and with the use of .our solutions, the resistance of the film decreasesperiodically and the film partially or .wholly dissolves, exposing new metal to the action of the electrolyte, thereby speeding up the .process and allowingmore metal to be removed .in.:a .shorter time with the same .amountmf wattage consumed.

That is to say, without film formation the at- TYPE OF CATHODE Any suitable material may be used as a cathode 1 provided it is not attacked nor excessively plated by the solution. Copper seems to be one of the best materials. It has a long life in the solution and good conductivity. Even copper cathodes have shown some attack at solution level but this can be eliminated by suitable insulation for two or three inches above and below the solution level.

DISTANCE CATHODE TO ANODE The distance between anode and cathode affects the time necessary to polish but this is functionally the same as increasing or decreasing the current density. In actual practice a distance of two to five inches between anode and cathode is practical. Conforming cathodes can be used when some deeply recessed piece is to be polished or when selective polishing is desired.

TIME, TEMPERATURE AND CURRENT DENSITY REQUIRED Time, temperature and current density depend on the material being polished. Acceptable finishes can be obtained on 2B stainless steels of the 321-302-304 grades in from three and onehalf to ten minutes at temperatures of 60-'75 0., with current densities of 50 to 250 amps per sq. ft., pressure 3.5 to '7 volts.

The time required to polish depends also on the condition of the material. The coarse rainy materials such as mill stock require longer times than the various semi-finished grades such as the B grades.

Increases in temperature with corresponding increases in current density reduce the time necessity to produce acceptable finishes.

Temperatures used, however, may be varied between 48 to 85 degrees Centigrade and the optimum temperature is between 60 to 65 degrees. volts, 50 to 500 amps per sq. ft. Increases in temperature require increases in current density but result in faster polishing. Agitation of the anode is desirable and speeds of from to 60 ft. per minute can be used. Agitation helps to minimize gas streaks.

MISCELLANEOUS Sludge formation in the operation of our processes and with the use of the preferred compositions is what normally can be expected in operations of this type. However, the sludge is well packed, falls to the bottom and can be easily removed. It doesnt cake or become hard and is easily removed from cathodes by immersion in water. Bath constituents can be varied and temperature and current density are no more critical than ordinary electroplating processes. The so- Current density should be from 4 to 9 6 lutions have long life and will not freeze due to crystallization.

These solutions will also polish nickel and nickel electroplates to mirror brightness and will also remove nickel from brass, copper,and their alloys very rapidly with little or no attack on the base metal. In such operations high current densities and high temperatures are desirable.

While I prefer to vary the current flow automatically by the use of electrolytic compositions which, during the polishing operations, form films which periodically build up and thin out or fall away, yet I realize that it is possible to vary the current flow in other ways and thus obtain some of the advantages of my new process in more eflicient polishing of metals. Therefore, I consider that while such mechanical current varying proc esses are not preferred by me, yet they do come under my inventions as disclosed herein.

While the forms of embodiments of the present invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow:

I claim:

1. A new composition of matter useful as an electrolytic solution for the anodic polishing of metal consisting by volume of a mixture of the equivalent of 40 to 65% of a 66 Baum sulfuric acid; the equivalent of from 5 to of a 26% aqueous solution of fluoboric acid; and from to water.

2. A composition of matter useful as an electrolytic solution for the anodic polishing of metal consisting by volume of the equivalent of 45% of a 66 Baum sulfuric acid, the equivalent of 13% of a 26% fluoboric acid, and 42% water.

3. A composition of matter useful for the anodic polishing of metal consisting by volume of the equivalent of a mixture of about 48% of a 66 Baume' sulfuric acid, the equivalent of about 14% of a 26% aqueous solution of fluoboric acid, and about 37% water together with a small amount of oxalic acid.

4. A process for the anodic polishing of a metal selected from the group consisting of iron, steel, nickel, and nickel electroplates which comprises immersing the metal in an electrolytic solution consisting of a mixture which is by volume the equivalent of a mixture of from 40% to of an aqueous solution of sulfuric acid of 66 Baum, the equivalent of from 5 to 20% of a 26% aqueous solution of fluoboric acid and the remainder mainly of water; and passing a direct electrical current from the metal to be cleaned through the solution and to a cathode immersed in the solution.

WILLIAM J. NEILL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,153,060 Guthrie Apr. 4, 1939 2,414,326 Newsome Jan. 14, 1047 FOREIGN PATENTS Number Country Date 530,041 Great Britain Dec. 4, 1940 OTHER REFERENCES "Metal Finishing, February 1947, pages 63-67. 

1. A NEW COMPOSITION OF MATTER USEFUL AS AN ELECTROLYTIC SOLUTION FOR THE ANODIC POLISHING OF METAL CONSISTING BY VOLUME OF A MIXTURE OF THE QUEIVALENT OF 45 TO 65% OF A 66* BAUME'' SULFURIC ACID; THE EQUIVALENT OF FROM 5 TO 20% OF A 26% AQUEOUS SOLUTION OF FLUOBORIC ACID; AND FROM 25 TO 55% WATER. 